Trump immigration ban

Overnight people with valid visas cannot re-enter the country - including people with green cards. Later rescinded for green cards holders, but only by a decisions of the Homeland Security secretary, that presumably can change his mind at any time.

Selfishly one could think that this applies only to particular 'special' (so to speak) countries.

Beside being selfish - what happens if a European citizen commit a terrorist act in the US? There are for sure some willing French? Belgian? maybe Italians?

Travel ban for Europeans then - including those with visas? Or travel ban for only one European country - followed by tit-for-tat retaliations from Europe ending up in complete travel freeze?

Yes, not very probable - but at this point who knows? Who can exclude such things?